Chromosome aberrations were studied in cultured chinese hamster cells on the 1--4th days after infection with Simian Virus 40. In the first and second mitoses after infection a statistically significant increase of the percentage of aberrant metaphases was observed (up to 22% as compared to 8--10% in intact cells). Already after 3 days following treatment the percentage of aberrant metaphases decreased sharply, reaching the control level. The virus induced a significant increase of the frequency of chromosome and chromatid breaks, as well as the appearance of fragments of an unknown origin. Chromosome breaks were distributed randomly among 5 morphologically distinct chromosome groups, according to the comparative length of respective chromosomes. An increase of the frequency of gaps and coiling deficiencies was also observed in virus-infected cultures. An earlier appearance of the first mitosis was observed in virus-treated cultures. It is suggested that the mutagenic effect of SV40 in the cells studied may be related to mechanisms controlling the integration of viral genomes into the cell chromosomes.